In a cellular communications system, a mobile radio station communicates over an assigned radio channel with a radio base station. Several base stations are connected to a switching node which is typically connected to a gateway that interfaces the cellular communications system with other communication systems. A call placed from an external network to a mobile station is directed to the gateway, and from the gateway through one or more switching nodes to a base station which serves the called mobile station. The base station pages the called mobile station and established a radio communications channel. A call originated by the mobile station follows a similar path in the opposite direction.
In a Code Division Multiple Access (CDMA) mobile communication system, the information transmitted between a base station and a mobile station is modulated using a spreading code. Spreading codes are used to distinguish information associated with different mobile stations or base stations transmitting over the same radio frequency band. In other words, individual radio "channels" correspond to and are discriminated on the basis of these codes. Various aspects of CDMA are set forth in one or more textbooks such as Applications of CDMA and Wireless/Personal Communications, Garg, Vijay K. et al., Prentice-Hall 1997.
Spread spectrum communications permit mobile transmissions to be received at two or more ("diverse") base stations and processed simultaneously to generate one received signal. With these combined signal processing capabilities, it is possible to perform a handover from one base station to another, (or from one antenna sector to another antenna sector connected to the same base station), without any perceptible disturbance in the voice or data communications. This kind of handover is typically called diversity handover.
During diversity handover, the signaling and voice information from plural sources is combined in a common point with decisions made on the "quality" of the received data. In soft handover, as a mobile station involved in a call moves to the edge of a base station's cell, the adjacent cell's base station assigns a transceiver to the same call while a transceiver in the current base station continues to handle that call as well. As a result, the call is handed over on a make-before-break basis. Soft diversity handover is therefore a process where two or more base stations handle the call simultaneously until the mobile station moves sufficiently close to one of the base stations which then exclusively handles the call. "Softer" diversity handover occurs when the mobile station is in handover between two different antenna sectors connected to the same, multisectored base station using a similar make-before-break methodology.
Because all users of a CDMA communications system transmit information using the same frequency band at the same time, each user's communication interferes with the communications of the other users. In addition, signals received by a base station from a mobile station close to the base station are much stronger than signals received from other mobile stations located at the base station's cell boundary. As a result, distant mobile communications are overshadowed and dominated by closein mobile stations which is why this condition is sometimes referred as the "near-far effect. "
Therefore, to achieve increased capacity by decreasing unnecessary interference, all mobile-transmitted signals should arrive at the base station with about the same average power irrespective of their distance from the base station. Transmit power control (TPC) is therefore a significant factor in improving the performance and capacity of a CDMA system. In general, the mobile station attempts to control its transmit power based on the power control messages sent to the mobile station from the base station with the end goal of controlling power received at the base station within a relatively small tolerance, e.g., 1 dB for all mobile station transmissions received at that base station.
The above power control operations relate to uplink (or reverse) power control for transmissions from the mobile station to the base station. Downlink (or forward) transmit power control is also important for transmission from a transceiver in the base station to the mobile station. In downlink power control, the base station varies the power of the transceiver transmitting to the mobile station depending on downlink transmit power control messages or commands sent by the mobile station.
Because power control in CDMA systems is very important, transmit power control adjustments occur very frequently, e.g., every 0.625 milliseconds. During downlink transmit power regulation, the mobile station is continually measuring the transmit power level received from the base station and determining whether the measured value is higher than a reference value. If so, a transmit power control bit having one value is sent uplink from the mobile station instructing the base station to decrease its transmit power to the mobile station by a predetermined increment, e.g., 1 dB, down to a minimum transmit power value. On the other hand, when the measured value is lower than the reference value, the transmit power control bit with the opposite value is transmit uplink to the base station for it to increase its transmit power by a predetermined increment, e.g., 1 dB, up to a maximum value. Downlink transmit power control begins while uplink and downlink synchronization are being acquired and continues throughout the mobile communication.
There are several problems with downlink transmit power among base stations or among base station sectors involved in diversity handover. Consider the example where a mobile station is traveling away from base station A toward base station B and is entering into a handover situation. Both base stations A and B receive the same downlink transmit power control command from the mobile station, and therefore, both base stations A and B transmit at a relatively high level to the mobile station for the duration of the handover operation irrespective of when or whether the mobile station receives one of the base station transmissions much more strongly (hereafter the "dominant" base station). In other words, it is usually unnecessary and counterproductive to have a less dominant base station transmit at high power levels to the mobile station until the mobile moves closer and/or the less dominant base station becomes more dominant. In effect, that high power level is wasted because the mobile is still essentially under the control of the dominant base station. Moreover, that high power level adversely interferes with other mobile communications.
To solve these problems, open loop power control is employed in the downlink power control during soft or softer handover. The open loop power control works in conjunction with closed loop power control and reduces the transmit power from the less dominant base stations or base station sectors involved in the soft or softer handover, e.g., those receiving signals from the mobile signal with lowest signal-to-interference ratio (SIR), thereby reducing interference to the system. The open loop control method in accordance with the present invention effectively and accurately controls the transmit powers from the base stations or sectors involved in the diversity handover so that only the dominant base station or sector is transmitting to the mobile station at a relatively high power. The transmit power levels of less dominant base stations or sectors are maintained at lower power levels to reduce interference. In addition, the handing over, dominant base station or sector decreases its transmit power to the mobile relatively quickly as soon as a less dominant base station or sector becomes the dominant base station or sector in the handover operation.
Accordingly, the present invention provides a method for controlling the transmit power of a base station transmission to a mobile station. The base station receives a signal from the mobile station and determines a signal-to-interference ratio (SIR) associated with the received signal. The base station controls its transmit power to the mobile station using the determined SIR value in conjunction with a transmit power control command received from the mobile station. When the mobile station is in the process of a soft handover involving two or more base stations, each of those base stations determines an SIR value associated with a signal received from the mobile station. Moreover, each of the base stations controls its respective power using both the power control command received from the mobile station and the SIR determined for that base station. Similarly, when the mobile station is in the process of softer handover involving two or more sectors of a single base station, the SIR associated with a signal received from the mobile station in each of those base station sectors is determined and used along with a power control command from the mobile station to control the respective power of each base station sector in conjunction with a power control command received from the mobile station.
In soft handover and softer handover applications of the invention, the mobile station signal is initially received at a first, dominant base station/sector with an SIR greater than the SIR of the mobile signal received at a second, less dominant base station/sector. As the mobile station moves into a soft/softer handover region between the first and second base stations/sectors, the SIR of the mobile signals received at the second base station/sector increases, and as a result, the transmit power to the mobile station at the second base station/sector increases. At the same time, the SIR of the mobile signals received at the first base station decreases, and accordingly, the transmit power to the mobile station from the first base station/sector decreases. The rate at which the transmit power for the first base station/sector decreases as well as the rate at which the transmit power from the first base station/sector increases may be selectively adjusted.
In a preferred embodiment, plural transmit power control commands from the mobile station are accumulated at each base station/sector (closed loop control), and the accumulated sum is combined with the determined SIR (open loop control). The SIR value may be preferably averaged or delayed and processed using a non-linear operation/function/scaling operation. The combined signal is used to optimally regulate the transmit power of the base station/sector to the mobile station.